1. Field of the Invention
The present invention relates to a polarization independent semiconductor optical amplifier or amplifying apparatus for amplifying a light signal or the like that is transmitted in an optical communication, and an optical communication system or network using the optical amplifier.
2. Related Background Art
An optical amplifier or amplifying apparatus for amplifying a light signal without using any opto-electric and/or electro-optic conversions is widely used in optical communication systems. Such an optical amplifier is a key device which is indispensable for the prolongation of relay distances for an optical transmission.
As a means for attaining an optical amplifier, a method of utilizing nonlinear scattering in an optical fiber, such as the Raman or the Brillouin effect, can be used. However, a traveling wave type optical amplifier, which uses the gain function of a semiconductor laser (LD), has been widely studied since it has the advatages of being compact, using a small amount of electric power, having structural simplicity and so forth. The traveling wave type optical amplifier can be expected to have characteristics such as its wavelength band of gain range being wide and that it can obtain a stable state of operation, regardless of variations in temperature.
Generally, a traveling wave type optical amplifier has a structure in which antireflection films are respectively formed on opposite end surfaces of a buried-structure laser diode (LD) and its active layer has typically a thickness of 0.1 .mu.m and a width of 2 .mu.m. As a result, the gain of such a traveling wave type optical amplifier is largely dependent on polarization due to its asymmetry in thickness and the width of the active layer. Thus, an optical amplifier of this type exhibits a difference in gain of several decibels for light components having different polarization states.
The polarization state of a light signal that is propagated through an optical fiber would be fluctuating. Therefore, an error is caused at the time of signal receiving if the gain of an optical amplifier is strongly dependent on polarization, as discussed above. In general, although the waveguide structure of an active layer only has to be equalized (its thickness is increased and its width is narrowed) in order to reduce the dependency of gain on polarization for incident light, the thickness and width need to be set below 0.5 .mu.m.times.0.5 .mu.m to keep a single mode condition. However, it is difficult to achieve such a size using present technologies.